Urban explorers, the prologue to "After..." tells us, are "extreme sport's answer to computer hackers," breaking their way into the hidden corners of our cities just for the thrill of it. There's a terrific horror movie to be made out of the very idea of these adventurers, considering the claustrophobia, the mystery, the very foreignness of such secluded areas - so close to modern society, yet so far removed. "After..." isn't that movie, as it goes a little too far off its own tracks, but it's a decent attempt.

It helps that director/co-writer David L. Cunningham ("To End All Wars") shot much of his film in the same guerilla style his characters portray. A portion of the movie was shot illegally in Moscow; one key sequence features a genuine nighttime BASE jump; the whole thing was shot using small HD video cameras, allowing for filming in tight spaces. The whole thing gives the movie a tremendously grimy "outlaw" look, which is then heightened by spastic editing and editing gimmickry that enhances the raw video-ness of some of the footage. The capper is a constant, brooding soundtrack courtesy electronica thumpers The Crystal Method.

But the film ultimately isn't about urban exploring at all; it merely uses such a notion as a springboard for something entirely different. This may obviously upset some viewers who go in to the film hoping for something along the lines of "The Descent" by way of "Raw Meat" and leave baffled by all the unanswered questions and admittedly ridiculous psychoanalysis. Yet different is good, and while "After..." ultimately leads to a derivative ending (without giving things away, genre fans will recognize where the plot's heading long before it gets there, thanks to an overwhelming similarity to a certain modern classic), the journey itself is fascinating and fresh - and moody as hell.

Three adventurers - Nate (Daniel Caltagirone), his wife Addy (Flora Montgomery), and her brother Jay (Nicholas Aaron) - have bribed their way into Moscow, where they hope to explore centuries' worth of underground mysteries, including such legends as Stalin's secret Metro-2 line and Ivan the Terrible's torture chamber. Flashbacks reveal that Nate and Addy are recovering from personal tragedy, and Nate especially needs this trip to help him recover.

One flashback, featuring a tense break-in, climb, and eventual jump, opens the film, while the first scene involving the Moscow subway system delivers overwhelming tension as the characters struggle to avoid getting crushed by the passing trains. Both set the stage for something tight and nerve-racking, and such a mood is kept up for the next half hour or so.

But then Cunningham and co-writer Kevin Miller shift gears, turning the suspense into something more internal. As we encounter a horde of vagrants dying from some unexplained radiation phenomenon and a roving gang of gun-toting militants who are apparently hunting them (or something - from here on in, everything becomes more confused), the action becomes increasingly cross-cut with flashbacks, apparent dream sequences, and effects shots that briefly but noticeably warp reality. Before too long, Nate's on a subway train, watching his own life play out on video monitors placed throughout the car.

So what exactly is going on? Even knowing the ending in advance doesn't quite provide enough explanations, which is wearisome, to be sure. And the constant fiddling with reality leaves us sometimes dismissing any concern for the characters, as who's to say if what's happening in a given scene is actually happening? It's difficult to become invested in people who may not actually be doing what they seem to be doing. It's frustrating.

But it's also fascinating. Scenes work on their own to create an inescapable eeriness, and together they build to something altogether chilling. And while the movie never quite hits the highs the concept promises, it's still an effective little thriller.